Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated

[Vincent Guittot]

On Tue, 14 Mar 2023 at 13:07, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Tue, Mar 14, 2023 at 08:41:30AM +0100, Vincent Guittot wrote:
>
> > I'm going to use something a bit different from your proposal below by
> > merging initial and flag
> > static void place_entity(struct cfs_rq *cfs_rq, struct sched_entity
> > *se, int flags)
> >
> > with flags:
> > 0 for initial placement
> > ENQUEUE_WAKEUP for wakeup
> > ENQUEUE_MIGRATED for migrated task
>
> So when a task is not running for a long time (our case at hand), then
> there's two cases:
>
>  - it wakes up locally and place_entity() gets to reset vruntime;
>  - it wakes up remotely and migrate_task_rq_fair() can reset vruntime.
>
> So if we can rely on ENQUEUE_MIGRATED to differentiate between these
> cases, when wouldn't something like this work?
>
> ---
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 7a1b1f855b96..a0d00b6a8bc6 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4648,11 +4648,31 @@ static void check_spread(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  #endif
>  }
>
> +static bool reset_vruntime(struct cfs_rq *cfs_rq, struct sched_entity *se)
> +{
> +       const s64 limit = 60LL * NSEC_PER_SEC;
> +       s64 sleep_time;
> +
> +       /*
> +        * Pull vruntime of the entity being placed to the base level of
> +        * cfs_rq, to prevent boosting it if placed backwards.  If the entity
> +        * slept for a long time, don't even try to compare its vruntime with
> +        * the base as it may be too far off and the comparison may get
> +        * inversed due to s64 overflow.
> +        */
> +       sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start;
> +       if (unlikely(sleep_time > limit)) {
> +               se->vruntime = cfs_rq->min_vruntime - calc_delta_fair(limit, se);
> +               return true;
> +       }
> +
> +       return false;
> +}
> +
>  static void
>  place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
>  {
>         u64 vruntime = cfs_rq->min_vruntime;
> -       u64 sleep_time;
>
>         /*
>          * The 'current' period is already promised to the current tasks,
> @@ -4682,18 +4702,8 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
>                 vruntime -= thresh;
>         }
>
> -       /*
> -        * Pull vruntime of the entity being placed to the base level of
> -        * cfs_rq, to prevent boosting it if placed backwards.  If the entity
> -        * slept for a long time, don't even try to compare its vruntime with
> -        * the base as it may be too far off and the comparison may get
> -        * inversed due to s64 overflow.
> -        */
> -       sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start;
> -       if ((s64)sleep_time > 60LL * NSEC_PER_SEC)
> -               se->vruntime = vruntime;
> -       else
> -               se->vruntime = max_vruntime(se->vruntime, vruntime);
> +       /* ensure we don't gain time by being placed backwards */
> +       se->vruntime = max_vruntime(se->vruntime, vruntime);
>  }
>
>  static void check_enqueue_throttle(struct cfs_rq *cfs_rq);
> @@ -4768,8 +4778,11 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
>         update_cfs_group(se);
>         account_entity_enqueue(cfs_rq, se);
>
> -       if (flags & ENQUEUE_WAKEUP)
> +       if (flags & ENQUEUE_WAKEUP) {
> +               if (!(flags & ENQUEUE_MIGRATED))
> +                       reset_vruntime(cfs_rq, se);
>                 place_entity(cfs_rq, se, 0);
> +       }
>
>         check_schedstat_required();
>         update_stats_enqueue_fair(cfs_rq, se, flags);
> @@ -7625,6 +7638,7 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>  static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
>  {
>         struct sched_entity *se = &p->se;
> +       struct cfs_rq *cfs_rq = cfs_rq_of(se);
>
>         /*
>          * As blocked tasks retain absolute vruntime the migration needs to
> @@ -7632,11 +7646,8 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
>          * min_vruntime -- the latter is done by enqueue_entity() when placing
>          * the task on the new runqueue.
>          */
> -       if (READ_ONCE(p->__state) == TASK_WAKING) {
> -               struct cfs_rq *cfs_rq = cfs_rq_of(se);
> -
> +       if (READ_ONCE(p->__state) == TASK_WAKING || reset_vruntime(cfs_rq, se))

That's somehow what was proposed in one of the previous proposals but
we can't call rq_clock_task(rq_of(cfs_rq)) because rq lock might not
be hold and rq task clock has not been updated before being used

>                 se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);
> -       }
>
>         if (!task_on_rq_migrating(p)) {
>                 remove_entity_load_avg(se);